`sockaddr_pppox` is a packed struct, which godefs doesn't understand.
So, I had to do the byte packing by hand instead.

Change-Id: Ib92cf43f8ae0729a0af043c4241063911c95e6bf
GitHub-Last-Rev: 8982dec099594064d3892f3fa05bb7b5989cec6d
GitHub-Pull-Request: golang/sys#23
Reviewed-on: https://go-review.googlesource.com/c/149757
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>